* French special forces facing "harassment" attacks -officer
* Gao action reveals rapid French advance against Islamists
* Islamist fighters pull back, flee French air strikes
* African force struggles with logistics, seeks help (Recasts with details of French operation at Gao, background.)
By Richard Valdmanis
KONNA, Mali, Jan 26 (Reuters) - French special forces in Mali with air support on Saturday seized the airport and a key bridge over the Niger River at the Islamist rebel-held stronghold of Gao as France accelerated its ground offensive against al Qaeda-allied fighters.
"The rebels have melted in to the local population. There is harassment. The operation is still under way. It is a bit complicated," a French officer in Mali, who asked not to be named, told Reuters, referring to the assault on Gao.
France's Defence Minister Jean-Yves Le Drian announced the seizure of the airport and bridge at Gao, the largest town in Mali's Saharan north which was occupied last year by a coalition of Islamist groups including al Qaeda's North African wing AQIM.
France's defence ministry initially gave few details of the operation at Gao, but there were unconfirmed reports from Malian sources that it involved French paratroops.
The French officer said the attacking French forces were facing "harassment" attacks but no solid line of resistance.
The speed of the French action at Gao suggested French and Malian governmnent troops intended to drive aggressively into the north of Mali in the next few days against other Islamist rebel-held towns, such as Timbuktu and Kidal.
For two weeks, French jets and helicopter gunships have been harrying the retreating Islamists, destroying their vehicles, command posts and weapons depots. The French action had already halted a sudden Islamist offensive launched in early January that had threatened Mali's southern capital Bamako.
A French defence ministry statement quoted minister Le Drian as saying that many of the Islamist fighters' vehicles and logistics bases had been destroyed.
News that the French forces were at Gao came as African states struggled to deploy a planned 6,000-strong African intervention force in Mali, known as AFISMA, under a U.N. mandate.
African Union leaders meeting at a summit in Addis Ababa were calling on the United Nations to provide emergency logistics and funding to allow the AFISMA force to deploy.
AU officials say AFISMA is severely hampered by logistical shortages and needs airlift support, ammunition, telecoms equipment, field hospitals, food and water. It also required training to operate in Mali's desert and arid mountains.
So far, only around 1,200 soldiers of the African force, to be mostly comprised of troops from neighbouring West African nations, have arrived in Mali.
In contrast, France has 2,500 soldiers already on the ground in its former colony, taking the lead in the offensive against the Islamist groups.
The United States and Europe strongly back the U.N.-mandated Mali operation as a counterstrike against the threat of radical Islamist jihadists using the country's inhospitable Saharan north as a launch pad for international terrorist attacks.
But Washington and European governments, while providing airlift and intelligence support to the operation, are not planning to send in any combat troops. (Reporting by James Regan in Paris, Richard Valdmanis in Konna, Mali and David Lewis in Dakar; Writing by Pascal Fletcher)